WASHINGTON -- A top U.S. military commander has announced more frequent rotations of U.S. combat units to Eastern Europe beginning early next year, citing an "aggressive Russia" as the main reason for the increase.

The announcement on March 30 by Air Force General Philip Breedlove, commander of U.S. military forces in Europe, was the latest in a series of moves by the Pentagon to bolster units and equipment on the continent, in an effort to reassure nervous NATO allies.

In a statement, Breedlove said an additional armored brigade would be begin rotating into Eastern Europe in February 2017, bringing the number of U.S. combat brigades that are constantly in the region to three. A brigade typically has between 4,200 and 4,700 troops.

The rotation "continues to demonstrate our strong and balanced approach to reassuring our NATO Allies and partners in the wake of an aggressive Russia in Eastern Europe and elsewhere," he said.

Earlier this year, U.S. Defense Secretary Ash Carter announced a quadrupling in U.S. spending -- $3.4 billion -- for more operations in Europe, under a plan called the European Reassurance Initiative.

Officials have said that this will include stationing some heavy equipment like tanks, combat vehicles, and artillery pieces on a permanent basis in the region.

NATO allies in Eastern Europe in particular have pushed hard for a bolstered military presence, in response to Russia's annexation of Ukraine's Crimean Peninsula and the conflict in eastern Ukraine.

There are about 62,000 U.S. military troops permanently assigned to Europe.